Foetal development is currently measured in a variety of ways by the midwives and clinicians supervising pregnancy. Current routine measures of foetal development include simple manual methods, such as physical examination by palpation, and auscultation of foetal heart noise; the mother's history, such as date of last menstrual period; the presence or absence of foetal movement ("quickening"); the size of the uterine enlargement; and total maternal weight gain.
These have now been supplemented by advanced technological methods. In particular, diagnostic ultrasound provides a high level of information about the foetus providing a "photographic" like real-time image of the foetus which allows the detection of gross physical abnormality. Diagnostic ultrasound is, however, semi-invasive in that a high frequency soundwave is transmitted into the foetus and the reflected waves are recorded according to how much is absorbed into the foetus or reflected back to the sensor. Although considered safe, there is some uncertainty about potential adverse effects of exposing the foetus to ultrasound over long periods.
Ultrasound use has now been extended to develop a clinical "biophysical profile" in which the heart rate is measured, the presence of foetal breathing movements is sought, and then the presence of spontaneous or evoked (e.g. by manual probing or by externally applied auditory stimuli) foetal body movements. By examining a foetus in this way the attending clinicians are reassured that it is developing normally, or in other cases is identified to be at risk, for example, of placental insufficiency. Although the level of information generated by ultrasound is highly useful and valuable, it provides only a "snapshot" (typically the longest duration being about 20 minutes) of foetus activity in any one day.
While ultrasound can detect abnormalities such as bradycardia and provide a guide to the presence of foetal distress, for example from placental insufficiency, it does not provide a ready means of for quantitatively determining foetal circulatory impedance on a continuous basis or at least over lengthy periods of time (e.g. greater than 20-30 minutes).
Circulation is, of course, a vital function as it constitutes the only means by which cells can receive oxygen and other materials needed for their survival. Similarly, the circulation effects removal of carbon dioxide and other waste products from cells. Blood circulates in the body for the same reason that any fluid flows, i.e. because of pressure gradients that exist in the body. For example, in the heart, blood circulates from the left ventricle to the right atrium of the heart because a blood pressure gradient exists between these two structures. For example, a typical normal blood pressure in the aorta, as the left ventricle contracts pumping blood into it, is 120 mmHg, and as the left ventricle relaxes, it decreases to 80 mmHg with the pressure in the arterial system progressively falling to 0 mmHg by the time blood reaches the venae cavae and right atrium. The progressive fall in pressure as blood passes through the circulatory system is directly related to the resistance offered to the blood flow by the circulatory system. For example, the greatest drop in pressure (about 50 mmHg) occurs across the arterioles because they present the greatest resistance to blood flow.
The primary determinant of arterial blood pressure is the volume of blood in the arteries. Other factors that determine arterial blood pressure include the cardiac output and peripheral resistance of the blood vessels. Cardiac output is determined by both the volume of blood pumped out of the ventricles by each beat (i.e. stroke volume) and by the heart rate. Stroke volume reflects the force or strength of ventricular contraction, i.e. the stronger the contraction, the greater the stroke volume tends to be.
Arterial blood pressure is routinely measured with the aid of a sphygmomanometer which makes it possible to measure the amount of air pressure equal to the blood pressure in an artery. The sphygmomanometer generally consists of a rubber cuff attached by a rubber tube to a compressible bulb and by another tube to a column of mercury that is marked off in millimeters. The cuff is wrapped around the arm over the brachial artery, and air is pumped into the cuff by means of the bulb. In this way, air pressure is exerted against the outside of the artery. Air is added until the air pressure exceeds the blood pressure within the artery and so compresses it. At this time no pulse can be heard through a stethoscope placed over the brachial artery at the bend of the elbow along the inner margin of the biceps muscle. By slowly releasing the air in the cuff the air pressure is decreased until it approximately equals the blood pressure within the artery. At this point the vessel opens slightly and a small spurt of blood comes through producing a first sound. This is followed by increasingly louder sounds that suddenly change. They became more muffled, then disappear altogether. Clinicians listen to these sounds whilst simultaneously reading the column of mercury. The first sound represents the systolic blood pressure. Systolic pressure is the force with which the blood is pushed against the artery walls when the ventricles are contracting. The lowest point at which the sounds can just be heard before they disappear is approximately equal to the diastolic pressure or the force of the blood when the ventricles are relaxed.
Systolic pressure gives valuable information about the force of the left ventricular contraction and diastolic pressure gives valuable information about the resistance of the blood vessels. Clinically, diastolic pressure is considered more important than systolic pressure because it indicates the pressure or strain to which blood vessel walls are constantly subjected. It also reflects the condition of the peripheral vessels since diastolic pressure rises or falls with the peripheral resistance. If for instance arteries are sclerosed, peripheral resistance and diastolic pressure both increase.
While outside the womb, it is readily straightforward to monitor the circulatory system of a human by taking such action as measuring blood pressure and pulse, it is not so straightforward to determine for the foetus in utero. The present invention is directed to one means of monitoring the circulatory condition of a foetus.